1. Field of the Invention
This invention relates to medical respirators, and more particularly to a respirator system and method having an interruptable breath assist mode in which delivery of breathing gas to a patient is initiated by the patient attempting to inhale.
2. Description of the Prior Art
Many respirators employ an "assist" mode to promote healthy patient breathing in which the patient's breath activity is monitored and a volume of air forced into his lungs when a breath attempt is detected. The assisted breaths are generally provided at steady rate until the patient is able to breathe on his own and it is safe to disconnect the respirator. These systems are capable of providing the patient with a sufficient amount of air to maintain a safe oxygen supply for as long as he is unable to breathe under his own power.
A problem may arise, however, in disconnecting the respirator. If the patient is left on his own too soon, he will be unable to breathe adequately and must be reconnected to the respirator without delay. On the other hand, the respirator may be left in place longer than is really necessary. In this case, in addition to any discomfort to the patient or unavailability of the apparatus for other patients that may be occasioned, the patient's return to unassisted breathing, which is generally a gradual process in which the patient builds up capacity breath by breath, may actually be retarded.
An alternation between spontaneous (i.e., a patient initiated and maintained positive pressure breath in which the patient controls all the parameters such as tidal volume, pulmonary pressure, flow rate, etc., except the blending of the breathing gases, i.e., the fractional inspriatory oxygen concentration (FIO.sub.2), whereas on the other hand, an assisted, sometimes referred to as simply as assist breath, is one delivered to a patient in response to an effort to breath put forth by a patient when he is too weak to maintain the effort so as to obtain a full volume of air unassisted. Thus, the ventilator in an assist mode controls all the breathing parameters except the time of delivery of the breath. A controlled, sometimes simply control, breath, in contrast to both spontaneous and assist breaths, is one in which all breathing parameters are controlled by the ventilator including the time of delivery of the breath.) and assisted breathing has been achieved in an existing system by providing a constant air stream from which the patient can draw breaths upon demand, and superimposing thereon an involuntary breath assist mechanism which forces a predetermined amount of air into the patient's lungs at regular intervals independent of the patient's spontaneous breathing. Such a system may be adapted to wean a patient away from an assisted breathing mode by gradually increasing the interval between forced breaths. However, it is unresponsive to the patient's minute-by-minute spontaneous breathing pattern, since an assisted breath is produced at the preset time regardless of the spontaneous breathing rate. In addition, there is a possibility of excess pressure being produced when an assisted breath is produced on top of a spontaneous breath, at which time the patient's lungs already contain an appreciable amount of air.
Other systems have been used to interrupt a regular pattern of assisted breaths, although not necessarily for the purpose of weaning a patient from the respirator. For example, a binary counter has been used to initiate a sigh (larger than normal air volume) breath after a series of 32, 64, or 128 assisted breaths of normal volume. While it has been found desirable to provide a "sigh" control in the respirator system to periodically break the constant breathing rate, such a mechanism does not contribute towards reducing the patient's dependency on the respirator.
In another prior art device alternate spontaneous and assisted breathing air conduits are provided. This respirator is normally operated in the assist mode, but the spontaneous breath conduit opens to override the assist mechanism if the patient begins to breathe on his own before the time for the next assisted breath. The device is most useful in the transition period when the patient first begins spontaneous breathing, but is of more limited help in the later stages of weaning a patient away from a respirator, since breath assistance is provided only if the patient fails to breathe spontaneously within a set time period.